258 



Fig. 6. Embryo, 42 hours old, showing a slightly more advanced stage in the formation 

 of the amniotic invagination and a beginning differentiation of the hydrocoel. 



7. Embryo, 30 hours old, showing the archenteron completely separated off from the 

 blastoporus. 

 - 8. Embryo, 2V4 days old, showing a further advanced stage of the amniotic invagina- 

 tion, which has formed a furrow nearly round the body. A distinct, broad ciUated 

 band is seen round the middle of the body. The archenteron has been separated 

 oflf from the blastoporus. The Internal structures very indistinct in this specimen. 



9. Embryo, 3V4 days old. The tubefeet of the young urchin have begun to protrude 

 through the amniotic invagination. The internal structures not to be made out 

 in this specimen. 



10. Embryo, 8^/4 days old. The primary tubefeet and the first spines (numbering 10, 

 both together, in this specimen) are protruding through the amniotic cavity, now 

 spht open. The pigment shows a rather distinct arrangement in bands. 



11. Embryo, 2^/4 days old, showing the five primary tubefeet lying within the amnion. 



12. Embryo, 41/2 days old. The primary tubefeet and spines have further protruded, 

 and the oral and aboral end of the larval body are pushed up on the dorsal side of 

 the urchin. 



13. Nearly metamorphosed sea-urchin; the aboral part of the larval body has been 

 nearly completely absorbed, forming only a small prominence on the upper side. 

 The primary tubefeet are fully extended. Embryonal spines have begun to appear 

 also on the aboral side. 



14. Embryo, 4V2 days old. The amniotic invagination has been widened so much that 

 the primary tubefeet and the spines are assuming a circular arrangement. Only 

 two of the tubefeet extended. The pigment forms a very prominent band above 

 the widening of the body. 



Figures 1 — 9 and 11 are drawn from specimens mounted in balsam and thus 

 made transparent; figures 10 and 12 — 14 are drawn from specimens not thus clear- 

 ed up. 



Plate XVIII. 



All figures of Heliocidaris erythrogramma (Val.). All, except fig. 4, i'"/i. 

 Fig. 1. Section of an embryo, 6 hours old, in a young cleavage stage. The formation of 

 the mesenchyme has begun. The nuclei very indistinct. 



2. Section of an embryo, 18 hours old, in a young blastula stage. The whole of the 

 blastocoel cavity filled by mesenchyme cells, still in an embryonal condition. 



3. Section of an embryo, 18 hours old, in the fully formed blastula stage. The mesen- 

 chyme cells have assumed the character of a reticulum, interwoven between the 

 large vacuoles, containing a probably fatty substance which serves as nourishment 

 to the developing embryo. 



4. Part of a longitudinal section of an embryo, 2V4 days old, showing the formation 

 of the amniotic invagination, ^""f^. 



5. Longitudinal section of an embryo, 27, days old; showing the primary tubefeet (t) 

 protruding into the amniotic cavity (am.), which opens out through a small pore, 

 c. enterocoel. 



6. Part of a section from the same series as fig. 5. The amnion is closed here, 

 h. hydrocoel. 



7. Longitudinal section of an embryo, 42 hours old. Showing the hydrocoel (h) lying 

 close towards the amniotic invagination (am), which is here still an open groove. 

 Trace of the blastoporus is seen in the oral end; ent. entoderm. 



8—9. Two longitudinal sections of an embryo in metamorphosis, 47^ days old. In 

 fig. 8 is seen the stone canal and its outer opening, in fig. 9 the inner opening of the 

 stone canal into the hydrocoel. a. ampulla; c. coelom; d. dorsal; ent. entoderm, 

 h. hydrocoel; r. reservoir of food; st. c. stone canal; v. ventral. 



